SummaryGrinding more than doubled the respiration rate of two silt loam soils, one arable and one grassland. The increases were smaller when the grinding treatment was given to portions of soils that had previously been fumigated with CHCI3and incubated, a treatment that greatly decreased microbial biomass.The results indicate that the flush of decomposition caused by grinding was in part derived from killed organisms and in part from enhanced decomposition of non‐biomass sections of the soil organic matter. Grinding killed about a quarter of the biomass in both soils. Carbon from killed organisms accounted for a quarter of the extra CO2–C evolved after grinding in the arable soil and almost half in the grassland soil. The extra non‐biomass organic matter decomposing after grinding amounted to about 0.5% of the soil organic carbon in both soils. This non‐biomass material rendered decomposable by grinding had a higher C/N ratio than the organic matter decomposing in ungrou